Menu Sign In Contact FAQ
Banner
Welcome to our forums

Help with VHF antenna identification

This is the COM2/NAV2 one on a TB20GT. I am sure it is a common type, but which one? There is some “corrosion” on it

Administrator
Shoreham EGKA, United Kingdom

Dorne-Margolin C 70 -3 Peter,come on!

Last Edited by MedFlyer at 11 Nov 16:16
LGGG

Hmmm, well found, thanks, Medflyer

The problem is that nobody I can find stocks it.

Claimed equivalents are C598501-0104, VF10-210, CI109 and CI121. The CI121 shows up at Adams.

Also I found this

which shows it to be 20.5" high while the CI121 is significantly shorter at 18.5"

Unsurprisingly the VSWR is 2.0 for the C70-3 but 2.5 for the CI121, and that doesn’t look like a good idea.

CI121 dimensions:

It does look like the mounting holes are in the same place for the DM70-3 and the CI121, but the CI121 will have a worse performance, won’t it?

Administrator
Shoreham EGKA, United Kingdom

Apologies for hi jacking this thread – I have three Comant antennas and they all look dirty/corroded. I have wiped them in WD40 several times, but they look naf. Any tips on bringing them back to original?

Norman
United Kingdom

Actually I did wonder about rubbing mine down and repainting it. It depends on whether there is solid metal there. If so, it would be trivial.

I suspect it is a conductor inside a fibreglass rod, like these, in which case this isn’t corrosion but just the paint coming off.

Is Dorne-Margolin French?

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

Is Dorne-Margolin French

I don’t know where it resided originally, but it has been American since at least 1992 when it was purchased by EDO, which is now called ITT Exelis and is a subsidiary of Harris.

LKBU near Prague, Czech Republic

Norman wrote:

I have wiped them in WD40 several times,

To rejuvanate both surfaces,remove the white salts by rubbing with fine sandpaper,wash,apply Alodine with a paintbrush,wait 1 hour,wash away,dry and you are ready for the next 3 years.After installing apply silicone around shoe AND over the screws.

LGGG

In the absence of any avionics installer input, @coolhand and @celtico might, I believe, know something about things like VSVR and whether the difference is relevant.

Administrator
Shoreham EGKA, United Kingdom

As being a Class 1 certified Radio Amateur HF operator,I can clean up some common grey areas:

-GA aircraft Com antennas are usualy of L/4 design.This design combines short length (which is desirable here) with a decent performance.
-These antennas beg for a good “ground”.Thats why we talk about clean metal surfaces and bondings.If no “ground” achieved,then you have poor performance and noise instead.
-The more vertical they are,the better.Inclined or bent types tend to intervene with aircraft body and produce higher SWR.Standing Wave Ratio idealy is 1:1 which is unachievable.That suggests that ALL emmited radiation is ejected out in space through the cables and the antenna.In the case of SWR 2,5:1 about 20% of the power is returning back to the Tranceiver ,heating it ! This Ratio is also considered tolerable in aviation.More is bad.
-One will ask why they cant manage aviation antennas with 1:1 SWR.The answer is they cant.You cant squeeze frequencies from 108 to 138 Mhz into a short fixed rod.You could if this rod could be made variable during flight (! like the old car antennas).Instead they “load” the short lenght of the rod with coil “reactance” which produces an average result.

Last Edited by MedFlyer at 12 Nov 11:03
LGGG

I was a licensed amateur too (OK1OFA, 1968-69) but I never learnt that stuff

So it sounds like the 2.5 antenna will be ok. I will try to clean up the existing one first though. One of the jobs for the Annual after xmas…

Administrator
Shoreham EGKA, United Kingdom
17 Posts
Sign in to add your message

Back to Top